Perhaps, on my DD-WRT WAP its pretty easy to tell who is in the house and who is next door with their signal strengths.

Maybe so, but what about the UPS guy whose smartphone has an uncannily good antenna?

Good question, how about a honey pot iPhone that you leave on the table and it calls the cops if that one stops being associated :-) Too bad you can't dial by IMEI, then you use use a RasPi pretending to be a cell tower to pull the IMEI and if it didn't recognize it, call the phone and say "please identify."

Ah the joys of a friday afternoon waiting for the next meeting to start ...